1. Field of the Invention
The invention relates to an electrical supply and starting system for a motor vehicle, and to a method for operating such a system.
2. Description of Related Art
Many motor vehicles have an internal combustion engine that is started by an integrated starter/generator system instead of by a conventional starter. The starter/generator system typically functions as a generator and also as a motor for starting the internal combustion engine. Small internal combustion engines (up to an engine capacity of approximately 2 liters) can be started by a starter/generator system of this kind with an on-board system voltage of 12 volts, such as an on-board system supplied from a conventional lead-acid battery. Larger motors require a higher voltage (for example 20 V) to provide the torque necessary to start an engine.
U.S. Pat. No. 6,995,480 B2 describes a power supply device for a motor vehicle having a motor/generator, an electronics system for driving the motor/generator, a conventional battery and a double-layer capacitor. The double-layer capacitor is connected directly to a DC voltage side of an inverter, and the battery is connected in parallel with the double-layer capacitor. Energy stored in the capacitor is used to start the internal combustion engine and electrical switches are used to disconnect the battery from the motor/generator system in specific situations.
DE 102 53 372 A1 describes an automatic machine starting system for machines having a large number of batteries. When an internal combustion engine of the machine is started, the batteries are connected in series to start the machine by supplying voltage of the batteries to a starter/generator of the machine starting system. While the machine is running, a control circuit, with the aid of a relay, connects the batteries back into a parallel circuit for charging purposes.
U.S. Pat. No. 3,463,995 describes an electrical power supply system for vehicles having an internal combustion engine, an electrical starter and further auxiliary batteries. The apparatus has a main battery with a high capacitance and auxiliary batteries with capacitances lower than the main battery. The various connections, relays and switches and the main battery are used in conjunction with the auxiliary batteries to start the internal combustion engine.
A starter/generator system for motor vehicles combines functions of a starter and a generator in a single electrical machine. The efficiency of a modern hybrid motor vehicle is increased by powerful starter/generators. More particularly, electrical energy is returned to the vehicle battery in a recuperative manner when the hybrid motor vehicle is braked, and this electrical energy can be used later. Vehicles with internal combustion engines that have a relatively large engine capacity require a relatively high voltage for operating the generator to provide the required torque for starting the engine.
Therefore, electrical buffer storage means with a higher voltage are required in such vehicles. High-capacitance double-layer capacitors usually are used in this case. Furthermore, operation at a relatively high voltage is also usual when the starter/generator system is operated in generator mode, so that the double-layer capacitors can be charged. However, this means a voltage converter for converting this relatively high voltage to 12 volts is required since the on-board system voltage of the vehicles has to continue to be kept at 12 volts to use cost-effective, conventional electrical components. Therefore, a high level of outlay is required overall for vehicles with relatively large internal combustion engines. This outlay being reflected in price and making the cost-benefit ratio problematic.
FIG. 2 shows a schematic block diagram of an exemplary electrical supply system for a motor vehicle.
In the schematic block diagram, a 12 V load 240 of the motor vehicle represents one or more 12 V loads of the motor vehicle and is connected firstly to the ground of the motor vehicle and secondly to the on-board electrical system of the motor vehicle. A 12 V lead-acid battery 232 has a negative pole connected to the ground of the motor vehicle, and also is connected to the on-board electrical system of the motor vehicle. The on-board electrical system of the motor vehicle also has a DC/DC converter 230 with a low-voltage side connected to the on-board electrical system of the supply system 250. The DC/DC converter 230 also has a high-voltage side connected to a high-voltage energy storage means 220 and to a further high-voltage load 225. The high-voltage side of the on-board electrical system also is connected to the starter/generator system 210. This electrical supply system provides an increased voltage for starting the internal combustion engine.
Against this background, the object of the present invention is to provide an improved supply system that simplifies the construction and operation of an on-board electrical system with a motor/generator system.